Negative electrode active material for electric device, negative electrode for electric device and electric device

ABSTRACT

A negative electrode active material for an electric device includes an alloy containing Si in a range from greater than or equal to 17% by mass to less than 90% by mass, Ti in a range from 10% by mass to 83% by mass exclusive, Ge in a range from 0% by mass to 73% by mass exclusive, and inevitable impurities as a residue. The negative electrode active material can be obtained with a multi DC magnetron sputtering apparatus by use of, for example, Si, Ti and Ge as targets. An electric device employing the negative electrode active material can achieve long cycle life, and ensure a high capacity and improved cycle durability.

TECHNICAL FIELD

The present invention relates to a negative electrode active materialfor an electric device generally serving as a secondary battery or acapacitor preferably used as a driving power source of a motor for usein, for example, an electric vehicle (EV) or a hybrid electric vehicle(HEV). The present invention also relates to a negative electrode, anelectric device and a lithium ion secondary battery using the negativeelectrode active material.

BACKGROUND ART

Various measures for reduction of emissions of CO₂ are being taken inorder to deal with atmospheric pollution and global warming. Inparticular, in the automobile industry, the reduction of emissions ofCO₂ is highly expected in association with the spread of electricvehicles and hybrid electric vehicles. Thus, development ofhigh-performance secondary batteries serving as driving power sources ofmotors for use in such vehicles, is actively being carried out. Since ahigher capacity and cycle property are particularly required for thesecondary batteries for driving motors, lithium ion secondary batterieshaving high theoretical energy are gaining increasing attention amongother types of secondary batteries.

The lithium ion secondary batteries are required to store a large amountof electricity in positive electrodes and negative electrodes by unitmass, in order to increase energy density of the lithium ion secondarybatteries. The lithium ion secondary batteries highly depend on activematerials used in the respective electrodes to fulfill such arequirement.

As one of measures to improve performance of lithium ion secondarybatteries, there is known a negative electrode active material capableof ensuring a high capacity and releasing stress caused by expansion andcontraction, and a battery using such a negative electrode activematerial, as proposed in Patent Literature 1. The negative electrodeactive material contains Si as a first element, Ge as a second element,and at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb as a thirdelement. Patent Literature 1 discloses that the content of Ge is in therange from 5 to 12 atom %, and the content of the third element is inthe range from 0.5 to 10 atom %, with respect to the content of Si.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2007-305424

SUMMARY OF INVENTION

However, the lithium ion secondary battery using the negative electrodeactive material disclosed in Patent Literature 1 shifts from anamorphous state to a crystalline state when Si is alloyed with Li. As aresult, the volume is greatly changed, which causes a reduction in cyclelife of the electrode. In addition, in the case of using the Si activematerial, the capacity of the lithium ion secondary battery has atrade-off relationship with cycle durability. Thus, the lithium ionsecondary battery is highly required to ensure a higher capacity andimproved durability concurrently.

The present invention has been accomplished in view of the problem inconventional negative electrode active materials. An object of thepresent invention is to provide a negative electrode active material foran electric device such as a lithium ion secondary battery capable ofsuppressing amorphous-crystal phase transition so as to extend cyclelife while ensuring a high capacity. Another object of the presentinvention is to provide a negative electrode including the negativeelectrode active material, and an electric device such as a lithium ionsecondary battery using the negative electrode including the negativeelectrode active material.

A negative electrode active material for an electric device according toan aspect of the present invention includes an alloy containing Si in arange from greater than or equal to 17% by mass to less than 90% bymass, Ti in a range from 10% by mass to 83% by mass exclusive, Ge in arange from 0% by mass to 73% by mass exclusive, and inevitableimpurities as a residue. A negative electrode for an electric deviceaccording to the present invention includes the negative electrodeactive material according to the present invention. An electric deviceaccording to the present invention includes the negative electrodeactive material according to the present invention or the negativeelectrode according to the present invention. A lithium ion secondarybattery may be a representative example of the electric device accordingto the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a ternary composition diagram showing a composition range ofan Si—Ge—Ti series alloy included in a negative electrode activematerial for an electric device according to the present invention,wherein alloy compositions prepared in each example are plotted.

FIG. 2 is a ternary composition diagram showing a preferable compositionrange of the Si—Ge—Ti series alloy included in the negative electrodeactive material for an electric device according to the presentinvention. FIG. 3 is a ternary composition diagram showing a morepreferable composition range of the Si—Ge—Ti series alloy included inthe negative electrode active material for an electric device accordingto the present invention.

FIG. 4 is a ternary composition diagram showing a still more preferablecomposition range of the Si—Ge—Ti series alloy included in the negativeelectrode active material for an electric device according to thepresent invention.

FIG. 5 is a ternary composition diagram showing the most preferablecomposition range of the Si—Ge—Ti series alloy included in the negativeelectrode active material for an electric device according to thepresent invention.

FIG. 6 is a schematic cross-sectional view showing an example of alithium ion secondary battery according to an embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a negative electrode active material for an electric devicewill be explained in detail while exemplifying a negative electrode fora lithium ion secondary battery and a lithium ion secondary batteryusing the negative electrode active material. It should be noted thatthe symbol “%” represents a percentage by mass unless otherwisespecified. In addition, dimensional ratios in the drawings are magnifiedfor convenience of explanation and may be different from actual ratios.

[Negative Electrode Active Material for Electric Device]

The negative electrode active material for a lithium ion secondarybattery according to an embodiment of the present invention will beexplained in detail below.

The negative electrode active material according to the presentinvention includes, as described above, an alloy containing Si withcontent in the range from greater than or equal to 17% by mass to lessthan 90% by mass, Ti with content in the range from 10% by mass to 83%by mass exclusive, Ge with content in the range from 0% by mass to 73%by mass exclusive, and inevitable impurities as a residue. Thesenumerical ranges correspond to the shaded area indicated in FIG. 1.

This negative electrode active material is used in a negative electrodefor an electric device such as a lithium ion secondary battery. In thiscase, an alloy contained in the negative electrode active materialabsorbs lithium ions when the battery is charged, and releases thelithium ions when the battery is discharged. The negative electrodeactive material contains Ge as a first additive element and Ti as asecond additive element that suppress amorphous-crystal phase transitionso as to extend cycle life when the negative electrode active materialis alloyed with lithium by charging. These additives contribute toensuring a higher capacity than conventional negative electrode activematerials, in particular, carbon series negative electrode activematerials. By optimizing the composition ranges of Ge and Ti as firstand second additive elements, the Si—Ge—Ti series negative electrodeactive material according to the present invention not only can ensure ahigh capacity but also can keep a high discharge capacity even after 50cycles or 100 cycles. Namely, the negative electrode active materialcontaining the Si—Ge—Ti series alloy with long cycle life can beobtained.

In the negative electrode active material containing the Si—Ge—Ti seriesalloy according the present invention, If the content of Si is less than17% by mass, the initial capacity of the battery tends to be decreased.In addition, if the content of Ti is 10% by mass or less, the cycle lifetends to be shortened.

In order to improve these properties, the alloy preferably contains Siwith content in the range from 17% by mass to 77% by mass, Ge withcontent in the range from 3% by mass to 63% by mass, and Ti with contentin the range from 20% by mass to 80% by mass as shown in the shaded areaof FIG. 2. The alloy more preferably contains Ti with content of 68% bymass or less as shown in the shaded area of FIG. 3. The alloy still morepreferably contains Si with content of 50% by mass or less as shown inthe shaded area of FIG. 4. The alloy most preferably contains Ti withcontent of 51% by mass or greater as shown in the shaded area of FIG. 5.

Note that the negative electrode active material according to thepresent invention inevitably contains impurities derived from the rawmaterials and the production method, in addition to the threecompositions described above. The content of the inevitable impuritiesis preferably less than 0.5% by mass, more preferably less than 0.1% bymass.

As described above, the alloy included in the negative electrode activematerial according to the present embodiment contains Si with content inthe range from greater than or equal to 17% by mass to less than 90% bymass, Ti with content in the range from 10% by mass to 83% by massexclusive, Ge with content in the range from 0% by mass to 73% by massexclusive, and inevitable impurities as a residue. Namely, the alloy iscomposed only of Si with content in the range from greater than or equalto 17% by mass to less than 90% by mass, Ti with content in the rangefrom 10% by mass to 83% by mass exclusive, Ge with content in the rangefrom 0% by mass to 73% by mass exclusive, and inevitable impurities as aresidue.

The method for manufacturing the negative electrode active material ofthe present invention, that is, the Si—Ge—Ti series alloy having theabove-described composition, is not particularly limited, and may be anyconventionally known method. That is, various manufacturing methods maybe used without any particular obstacle, since there is littledifference in the conditions and characteristics of the alloy producedby the manufacturing methods.

Specific examples of the method for manufacturing the thin film alloyhaving the above-described composition include a multi PVD method (asputtering method, a resistance heating method, a laser ablationmethod), and a multi CVD method (a chemical vapor-phase growth method).Examples of the multi PVD method include a sputtering method, aresistance heating method and a laser ablation method. The multi CVDmethod may be a chemical vapor-phase growth method. These manufacturingmethods can provide the negative electrode in a manner such that thealloyed thin film is directly formed on a current collector. Thus, thesemanufacturing methods contribute to simplification of the process. Inaddition, these manufacturing methods need not use other componentscomposing a negative electrode active material layer, such as a binderand an electric conducting additive, other than the alloy and therefore,the alloyed thin film as the negative electrode active material issimply used for the negative electrode. Accordingly, these manufacturingmethods contribute to a higher capacity and energy density, whichsatisfy the level suitable for practical use in vehicles, and aredesirable to examine electrochemical characteristics of the activematerial.

The method for manufacturing the alloyed thin film may use a multi DCmagnetron sputtering apparatus, such as an independently controllableternary DC magnetron sputtering apparatus. Such an apparatus can freelyform the Si—Ge—Ti series alloyed thin film having various alloycompositions and thicknesses, on the surface of the substrate (thecurrent collector). For example, the ternary DC magnetron sputteringapparatus uses target 1 (Si), target 2 (Ge) and target 3 (Ti), fixes thesputtering time, and changes the power level of the DC power source to185 W for Si, in the range from 0 to 120 W for Ge, and in the range from0 to 150 W for Ti. Accordingly, ternary series alloy samples havingvarious composition formulae can be obtained. Note that, sincesputtering conditions depend on sputtering devices, it is preferable toestimate appropriate ranges of the sputtering conditions throughpreliminary tests for each sputtering device.

As described above, the negative electrode active material layeraccording to the present embodiment can use the Si—Ge—Ti series alloyedthin film. Alternatively, the negative electrode active material layermay be a layer containing particles of the Si—Ge—Ti series alloy as amain component. Examples of the method for manufacturing such an alloyin a particle state having the composition described above include amechanical alloying method and an arc plasma melting method. When thealloy in the particle state is used as the negative electrode activematerial, slurry is prepared first in a manner such that a binder, anelectric conducting additive and a viscosity control solvent are addedto the alloy particles. The slurry thus obtained is then applied to thecurrent collector to form the negative electrode active material layer,so as to obtain the negative electrode. Such a process is superior interms of mass production and practicality for actual battery electrodes.

When the alloy in the particle state is used as the negative electrodeactive material, the average particle diameter of the alloy is notparticularly limited as long as it is substantially the same as that ofconventional negative electrode active materials. Here, the averageparticle diameter is preferably in the range from 1 μm to 20 μm in viewof higher output power, however, the average particle diameter may be inother ranges as long as it can achieve the effects described aboveappropriately.

Note that, in the description of the present invention, “the particlediameter” represents the greatest length between any two points on thecircumference of the active material particle (the observed plane)observed by observation means such as a scanning electron microscope(SEM) or a transmission electron microscope (TEM). In addition, “theaverage particle diameter” represents a value calculated with thescanning electron microscope (SEM) or the transmission electronmicroscope (TEM) as an average value of particle diameters of theparticles observed in several to several tens of fields of view.Particle diameters and average particle diameters of other constituentsmay be determined in the same manner.

[Negative Electrode for Electric Device and Electric Device]

The negative electrode for an electric device according to the presentinvention includes the negative electrode active material containing theSi—Ge—Ti series alloy. The lithium ion secondary battery as arepresentative example of the electric device includes at least onesingle cell including the negative electrode in which the negativeelectrode active material layers containing the negative electrodeactive material are provided on both sides of the current collector, thesingle cell further including an electrolyte layer and a positiveelectrode. Hereinafter, the constitution of the lithium ion secondarybattery and the materials used therein will be explained in detail.

(Constitution of Lithium Ion Secondary Battery)

FIG. 6 shows an example of the lithium ion secondary battery accordingto an embodiment of the present invention. As shown in FIG. 6, thelithium ion secondary battery 1 according to the present embodiment hasa constitution in which a battery element 10 to which a positiveelectrode tab 21 and a negative electrode tab 22 are attached, is sealedin an exterior body 30. In the present embodiment, the positiveelectrode tab 21 and the negative electrode tab 22 are exposed onopposite sides on the outside of the exterior body 30. Note that thepositive electrode tab and the negative electrode tab may be exposed onthe same side on the outside of the exterior body (not shown in thefigure). In addition, the positive electrode tab and the negativeelectrode tab may be attached to positive electrode current collectorsand negative electrode current collectors which will be described belowby, for example, ultrasonic welding or resistance welding.

(Positive Electrode Tab and Negative Electrode Tab)

The positive electrode tab 21 and the negative electrode tab 22 are madefrom a material such as aluminum (Al), copper (Cu), titanium (Ti),nickel (Ni), stainless steel (SUS), or an alloy thereof. However, thematerial is not limited to these, and may be any conventionally knownmaterial used for tabs for lithium ion secondary batteries. The positiveelectrode tab and the negative electrode tab may be made from the samematerial, or may be made from different materials. The tabs may beprepared preliminarily and connected to the positive electrode currentcollectors and the negative electrode current collectors which will bedescribed below, according to the present embodiment. Alternatively,each of the positive electrode current collectors and the negativeelectrode current collectors which will be described below may beextended to form the respective tabs when these are in a foil state.

[Exterior Body]

The exterior body 30 is preferably made from a film-like exteriormaterial in view of, for example, reduction in size and weight. However,the exterior body 30 is not limited to such a material, and may be anyconventionally known material available for exterior bodies for lithiumion secondary batteries. When the lithium ion secondary battery is usedfor a vehicle, a polymer-metal composite laminated sheet hiving highthermal conductivity is preferably used in order to transfer heatefficiently from a heat source the vehicle and rapidly heat the insideof the battery to a battery operation temperature.

(Battery Element)

As shown in FIG. 6, the battery element 10 in the lithium ion secondarybattery 1 according to the present embodiment includes plural singlecell layers 14 stacked on top of each other, each including a positiveelectrode 11, an electrolyte layer 13 and a negative electrode 12. Thepositive electrode 11 has a configuration in which positive electrodeactive material layers 11B are provided on both main surfaces of apositive electrode current collector 11A. The negative electrode 12 hasa configuration in which negative electrode active material layers 12Bare provided on both main surfaces of a negative electrode currentcollector 12A.

In this case, the positive electrode active material layer 11B providedon one main surface of the positive electrode current collector 11A ofthe positive electrode 11, faces the negative electrode active materiallayer 12B provided on one main surface of the negative electrode currentcollector 12A of the negative electrode 12 adjacent to the positiveelectrode 11, through the electrolyte layer 13. Several sets of thepositive electrode, the electrolyte layer and the negative electrodearranged in this order are stacked on top of each other. The positiveelectrode active material layer 11B, the electrolyte layer 13 and thenegative electrode active material layer 12B adjacent to each otherconstitute each single cell layer 14. Namely, the lithium ion secondarybattery 1 according to the present embodiment has a constitution inwhich the plural single cell layers 14 are stacked on top of each otherso as to be electrically connected in parallel. Here, the negativeelectrode current collectors 12A located on the outermost layers of thebattery element 10 are each provided with the negative electrode activematerial layer 12B only on one side thereof.

In addition, insulating layers (not shown in the figure) may be providedon peripheries of the respective single cell layers 14 to isolate thepositive electrode current collectors 11A and the negative electrodecurrent collectors 12A adjacent to each other. Such an insulating layerfor covering the single cell layer is preferably made from a materialthat can hold an electrolyte contained in the electrolyte layer andprevent liquid leakage of the electrolyte. In particular,general-purpose plastic such as polypropylene (PP), polyethylene (PE),polyurethane (PUR), polyamide resin (PA), polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVdF) and polystyrene (PS), may beused. Alternatively, thermoplastic olefin rubber or silicone rubber mayalso be used.

(Positive Electrode Current Collector and Negative Electrode CurrentCollector)

The positive electrode current collector 11A and the negative electrodecurrent collector 12A are made from an electrically conductive materialsuch as aluminum, copper and stainless steel (SUS) in a foil state or amesh state. However, the positive electrode current collector 11A andthe negative electrode current collector 12A are not limited to such amaterial, and may be any conventionally known material available forcurrent collectors for lithium ion secondary batteries. The size of thecurrent collectors may be determined depending on the intended use ofthe battery. For example, current collectors having large areas are usedfor a large-size battery for which high energy density is required. Thethickness of the current collectors is not particularly limited;however, the thickness is generally approximately in the range from 1 μmto 100 μm. The shape of the current collectors is not particularlylimited. The battery element 10 shown in FIG. 6 may use currentcollecting foils or mesh current collectors (such as expanded grids).The current collecting foils are suitable for use when the thin filmalloy as the negative electrode active material is directly formed onthe negative electrode current collector 12A by a sputtering method.

The material used for the current collectors is not particularly limitedas described above. Examples of the material include metal, and resin inwhich electrically conductive filler is added to an electricallyconductive polymer material or a non-conductive polymer material.Examples of the metal include aluminum, nickel, iron, stainless steel,titanium and copper. In addition, a clad metal of nickel and aluminum, aclad metal of copper and aluminum, or an alloyed material of thesemetals combined together, is preferably used. A foil in which the metalsurface is covered with aluminum may also be used. In particular,aluminum, stainless steel, copper and nickel are preferable in view ofelectron conductivity, battery action potential, and adhesion of thenegative electrode active material to the current collectors bysputtering.

Examples of the electrically conductive polymer material includepolyaniline, polypyrrole, polythiophene, polyacetylene,polyparaphenylene, polyphenylene vinylene, polyacrylonitrile andpolyoxadiazole. These electrically conductive polymer materials have anadvantage in simplification of the manufacturing process and lightnessof the current collectors, since these materials have sufficientelectric conductivity even if electrically conductive filler is notadded thereto.

Examples of the non-conductive polymer material include polyethylene(PE; such as high-density polyethylene (HDPE) and low-densitypolyethylene (LDPE)), polypropylene (PP), polyethylene terephthalate(PET), polyether nitrile (PEN), polyimide (PI), polyamide imide (PAI),polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber(SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride(PVdF) and polystyrene (PS). These non-conductive polymer materials havehigh potential tolerance or solvent tolerance.

The electrically conductive polymer material or the non-conductivepolymer material may include electrically conductive filler asnecessary. In particular, when the resin serving as a substrate of thecurrent collector only includes a non-conductive polymer, theelectrically conductive filler is essential to provide the resin withelectric conductivity. The electrically conductive filler is notparticularly limited as long as it is a substance having electricconductivity. Examples of the material having high electricconductivity, potential tolerance or lithium ion insulation, includemetal and electrically conductive carbon. The metal is not particularlylimited; however, the metal is preferably at least one element selectedfrom the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, Sband K, or an alloy or metal oxide containing these metals. Theelectrically conductive carbon is not particularly limited; however, thecarbon is preferably at least one material selected from the groupconsisting of acetylene black, Vulcan (registered trademark), BlackPearls (registered trademark), carbon nanofiber, Ketjenblack (registeredtrademark), carbon nanotube, carbon nanohorn, carbon nanoballoon andfullerene. The amount of the electrically conductive filler added in thecurrent collectors is not particularly limited as long as it providesthe current collectors with sufficient electric conductivity. Ingeneral, the amount is approximately in the range from 5% to 35% by massof the total.

However, the current collectors are not limited to the materialsdescribed above, and may be any conventional known material availablefor current collectors for lithium ion secondary batteries.

(Positive Electrode)

The positive electrode 11 of the lithium ion secondary battery has aconfiguration in which the positive electrode active material layers 11Bare formed on one surface or both surfaces of the positive electrodecurrent collector 11A made from an electrically conductive material suchas an aluminum foil, a copper foil, a nickel foil or a stainless. Thethickness of the positive electrode current collector is notparticularly limited as described above; however, it is generallypreferably approximately in the range from 1 μm to 30 μm.

The positive electrode active material layer 11B contains, as a positiveelectrode active material, any one of, or two or more positive electrodematerials capable of absorbing and releasing lithium, and may alsocontain an electric conducting additive and a binder as necessary. Thecontent ratio of the positive electrode active material, the electricconducting additive and the binder in the positive electrode activematerial layer, is not particularly limited.

Examples of the positive electrode active material include alithium-transition metal composite oxide, a lithium-transition metalphosphate compound, a lithium-transition metal sulfated compound, asolid solution series material, a ternary series material, an NiMnseries material, an NiCo series material and a spinel-manganese seriesmaterial.

Examples of the lithium-transition metal composite oxide includeLiMn₂O₄, LiCoO₂, LiNiO₂, Li(Ni, Mn, Co)O₂, Li(Li, Ni, Mn, Co)O₂ andLiFePO₄. In addition, an oxide in which part of the transition metalcontained in each of these composite oxides is substituted with otherelements, may be used. Examples of the solid solution series materialinclude xLiMO₂.(1-x)Li₂NO₃ (wherein 0<x<1, M represents at least onetransition metal element in an average oxidation state of 3+, and Nrepresents at least one transition metal element in an average oxidationstate of 4+), and LiRO₂—LiMn₂O₄ (R represents a transition metal elementsuch as Ni, Mn, Co or Fe).

The ternary series material may be a nickel-cobalt-manganese compositepositive electrode material. The spinel Mn series material may beLiMn₂O₄. The NiMn series material may be LiNi_(0.5)Mn_(1.5)O₄. The NiCoseries material may be Li(NiCo))O₂. In some cases, two or more positiveelectrode active materials may be used together. In view of having ahigher capacity and better output performance, the lithium-transitionmetal composite oxide is preferably used for the positive electrodeactive material.

The particle diameter of the positive electrode active material is notparticularly limited; however, it is generally preferably as small aspossible. The average particle diameter of the positive electrode activematerial may be approximately in the range from 1 μm to 30 μm, morepreferably approximately in the range from 5 μm to 20 μm, in view ofoperation efficiency and ease of handling. Of course, positive electrodeactive materials other than the above-described positive electrodeactive material may be used. In the case that the active materialsrequire different particle diameters in order to achieve their ownappropriate effects, the active materials having different particlediameters may be selected and mixed together so as to optimally functionto achieve their own effects. Thus, it is not necessary to equalize theparticle diameter of all of the active materials.

The binder is added to bind the active materials to each other or bindthe active material to the current collector to maintain the electrodestructure. The binder may be thermoplastic resin such as polyvinylidenefluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl acetate,polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polymethylacrylate (PMA), polymethyl methacrylate (PMMA), polyether nitrile (PEN),polyethylene (PE), polypropylene (PP) or polyacrylonitrile (PAN),thermoset resin such as epoxy resin, polyurethane resin or urea resin,or a rubber material such as styrene-butadiene rubber (SBR).

The electric conducting additive is also referred to as an electricconducting agent added to improve electric conductivity. The electricconducting additive used in the present invention is not particularlylimited, an conventionally known agent may be used. Examples of theelectric conducting additive include a carbon material such as carbonblack (such as acetylene black), graphite or carbon fiber. The additionof the electric conducting additive contributes to effectivelyestablishing an electronic network in the active material layer, andimproving output performance and battery reliability derived fromimprovement in retention of an electrolyte solution.

(Negative Electrode)

The negative electrode 12 has a configuration, as in the case of thepositive electrode, in which the negative electrode active materiallayers 12B are formed on one surface or both surfaces of the negativeelectrode current collector 12A made from the electrically conductivematerial as described above.

The negative electrode active material layer 12B contains, as a negativeelectrode active material, any one of, or two or more negative electrodematerials capable of absorbing and releasing lithium, and may alsocontain the same electric conducting additive and binder as the positiveelectrode active material as necessary. The content ratio of thenegative electrode active material, the electric conducting additive andthe binder in the negative electrode active material layer, is notparticularly limited.

The lithium ion secondary battery as the electric device according tothe present invention includes the negative electrode active materialcontaining, as an essential component, the Si—Ge—Ti series alloy havingthe above-described composition. As described above, the negativeelectrode active material layer 12B according to the present embodimentmay be a thin film including the Si—Ge—Ti series alloy. In this case,the negative electrode active material layer 12B may consist of theSi—Ge—Ti series alloy, or may further contain a conventionally knownnegative electrode active material capable of reversibly absorbing andreleasing lithium without any particular obstacle.

Alternatively, as described above, the negative electrode activematerial layer 12B may contain, as a main component, the particles ofthe Si—Ge—Ti series alloy. In this case, the negative electrode activematerial 12B may contain the electric conducting additive and thebinder, which may be contained also in the positive electrode activematerial layer 11B, as necessary. Note that, in the present description,“the main component” represents a component contained in the negativeelectrode active material layer 12B with content of greater than orequal to 50% by mass.

The other negative electrode active material used together may be acarbon material such as graphite that is highly crystalline carbon (suchas natural graphite or artificial graphite), low crystalline carbon(such as soft carbon or hard carbon), carbon black (such as Ketjenblack,acetylene black, channel black, lamp black, oil furnace black or thermalblack), fullerene, carbon nanotube, carbon nanofiber, carbon nanohorn orcarbon fibril. Examples of the negative electrode active materialfurther include a single substance alloyed with lithium such as Si, Ge,Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg,Ga, Tl, C, N, Sb, Bi, O, S, Se, Te or Cl, and an oxide and a carbidecontaining the elements listed above. Examples of the oxide includesilicon monoxide (SiO), SiO_(x) (0<x<2), tin dioxide (SnO₂), SnO_(x)(0<x<2) and SnSiO₃. The carbide may be silicon carbide (SiC). Otherexamples of the negative electrode active material include a metallicmaterial such as a lithium metal, and a lithium-transition metalcomposite oxide such as a lithium-titanium composite oxide (lithiumtitanate: Li₄Ti₅O₁₂). Each of these negative electrode active materialsmay be used singly, or two or more of these materials may be usedtogether.

Thus, the negative electrode may be obtained in a manner such thatslurry containing the negative electrode active material together withthe electric conducting additive and the binder, is applied to thesurface of the negative electrode current collector to form the negativeelectrode active material layer. Alternatively, the negative electrodemay be obtained in a manner such that the thin film of the negativeelectrode active material alloy is directly formed on the surface of thenegative electrode current collector by the multi PVD method or themulti CVD method.

As described above, the positive electrode active material layer and thenegative electrode active material layer are each provided on one sideor both sides of the respective current collectors. Alternatively, onecurrent collector may be provided with the positive electrode activematerial layer on one side, and provided with the negative electrodeactive material layer on the other side. Electrodes having such aconfiguration may be used for a bipolar battery.

(Electrolyte Layer)

The electrolyte layer 13 contains a non-aqueous electrolyte thatfunctions as a carrier of lithium ions that move between the positiveelectrode and the negative electrode at the time of charging anddischarging. The thickness of the electrolyte layer 13 is preferablyreduced as much as possible. The thickness is generally approximately inthe range from 1 μm to 100 μm, preferably in the range from 5 μm to 50μm.

The non-aqueous electrolyte contained in the electrolyte layer 13 is notparticularly limited as long as it functions as a carrier lithium ions,and may be a liquid electrolyte or a polymer electrolyte.

The liquid electrolyte has a constitution in which lithium salts(electrolyte salts) are dissolved in an organic solvent. The organicsolvent may be carbonate such as ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate (BC), vinylene carbonate (VC),dimethyl carbonate (DMC), diethyl, carbonate (DEC), ethyl methylcarbonate (EMC) or methyl propyl carbonate (MPC). The lithium salts maybe a compound that can be added to the electrode active material layers,such as Li(CF₃SO₂)₂N, Li(C₂F₅SO₂)₂N, LiPF₆, LiBF₄, LiAsF₆, LiTaF₆,LiClO₄ and LiCF₃SO₃.

The polymer electrolyte is classified into two types; a gel polymerelectrolyte (a gel electrolyte) containing an electrolysis solution, andan intrinsic polymer electrolyte not containing an electrolysissolution. The gel polymer electrolyte preferably has a constitution inwhich the liquid electrolyte is poured into a matrix polymer (a hostpolymer) including an ion conductive polymer. The use of the gel polymerelectrolyte decreases fluidity of the electrolyte so as to easilyinterrupt ion conduction between the respective layers.

The ion conductive polymer used as the matrix polymer (the host polymer)is not particularly limited, and examples thereof include polyethyleneoxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVDF),a copolymer of polyvinylidene fluoride and hexafluoropropylene(PVDF-HFP), polyethylene glycol (PEG), polyacrylonitrile (PAN),polymethyl methacrylate (PMMA), and a copolymer of these compounds.

The ion conductive polymer may be the same as or different from an ionconductive polymer used as the electrolyte in the active materiallayers, but it is preferably the same. The electrolysis solution(namely, the lithium salts and the organic solvent) is not particularlylimited, and may employ the electrolyte salts such as lithium salts andthe organic solvent such as carbonate as described above.

The intrinsic polymer electrolyte has a constitution in which lithiumsalts are dissolved in the matrix polymer, but no organic solvent iscontained. Thus, the use of the intrinsic polymer electrolytecontributes to reducing the risk of liquid leakage from the battery andthereby enhancing the reliability of the battery.

The matrix polymer of the gel polymer electrolyte or the intrinsicpolymer electrolyte can exhibit high mechanical strength when across-linked structure is formed. The cross-linked structure may beformed in a manner such that a polymerizable polymer used for polymerelectrolyte formation (for example, PEG or PPO) is subjected topolymerization by use of an appropriate polymerization initiator.Examples of the polymerization include thermal polymerization,ultraviolet polymerization, radiation polymerization and electron beampolymerization. The non-aqueous electrolyte contained in the electrolytelayer 13 may be used singly, or two or more kinds thereof may be mixed.

A separator is preferably used in the electrolyte layer 13 when theelectrolyte layer 13 contains the liquid electrolyte or the gel polymerelectrolyte. The specific configuration of the separator may be amicroporous film made from polyolefin such as polyethylene orpolypropylene.

(Battery Configuration)

The lithium ion secondary battery has a structure in which the batteryelement is housed in a battery case (a packing body) such as a can bodyor a laminated container. The battery element (the electrode structure)has a configuration in which the positive electrode and the negativeelectrode are connected to each other via the electrolyte layer. Thelithium ion secondary battery is mainly classified into two types: awound type battery including a battery element in which positiveelectrodes, electrolyte layers and negative electrodes are wound, and astacking type battery including a battery element in which positiveelectrodes, electrolyte layers and negative electrodes are stacked. Thebipolar battery described above corresponds to the stacking typebattery. The lithium ion secondary battery is also referred to as a coincell, a button battery or a laminated battery depending on the shape andstructure of the battery case.

EXAMPLE

Hereinafter, the present invention is explained in more detail withreference to examples; however, the present invention is not limited tothese examples.

[1] Preparation of Negative Electrode

As a sputtering apparatus, an independently controllable ternary DCmagnetron sputtering apparatus (manufactured by Yamato-Kiki IndustrialCo., Ltd.;

combinatorial sputter coating apparatus; gun-sample distance: about 100mm) was used. Thin films of negative electrode active material alloyshaving compositions according to the respective examples were eachformed on a current collector substrate made of a nickel foil having athickness of 20 μm under the following conditions. Accordingly, 31negative electrode samples were obtained

(Preparation Conditions)

(1) Targets (manufactured by Kojundo Chemical Laboratory Co., Ltd.;purity: 4N)

Si: diameter of 50.8 mm; thickness of 3 mm (backing plate of oxygen-freecopper with thickness of 2 mm)

Ge: diameter of 50.8 mm; thickness of 3 mm (backing plate of oxygen-freecopper with thickness of 2 mm)

Ti: diameter of 50.8 mm; thickness of 5 mm

(2) Conditions of Film Formation

Base pressure: up to 7×10⁻⁶ Pa

Sputtering gas: Ar (99.9999% or more)

Sputtering gas introduction amount: 10 sccm

Sputtering pressure: 30 mTorr

DC power source: Si (185 W), Ge (0 to 120 W), Ti (0 to 150 W)

Pre-sputtering time: 1 min.

Sputtering time: 10 min.

Substrate temperature: room temperature

In each example, the Si target, the Ge target and the Ti target wereused, the sputtering time was fixed at 10 minutes, and the power levelsof the DC power source were changed for each target within theabove-described ranges. Then, alloyed thin films in an amorphous statewere formed on Ni substrates, so as to obtain negative electrode samplesincluding the alloyed thin films having various compositions. Table 1and FIG. 1 to FIG. 5 show the element compositions of these alloyed thinfilms.

As for the sample preparation, for example, in Example 14, the DC powersource 1 (Si target) was set to 185 W, the DC power source 2 (Ge target)was set to 100 W, and the DC power source 3 (Ti target) was set to 130W. In Comparative Example 2, the DC power source 1 (Si target) was setto 185 W, the DC power source 2 (Ge target) was set to 100 W, and the DCpower source 3 (Ti target) was set to 0 W. In Comparative Example 9, theDC power source 1 (Si target) was set to 185 W, the DC power source 2(Ge target) was set to 0 W, and the DC power source 3 (Ti target) wasset to 40 W.

The obtained alloyed thin films were analyzed by using the followinganalysis method and analysis device:

(Analysis Method)

Composition analysis: SEM-EDX analysis (manufactured by JEOL Ltd.), EPMAanalysis (manufactured by JEOL Ltd.)

Film thickness measurement (for calculating sputtering rate): filmthickness meter (manufactured by Tokyo Instruments, Inc.)

Film state analysis: Raman spectroscopic analysis (manufactured byBruker Japan Co., Ltd.)

[2] Preparation of Batteries

Each negative electrode sample obtained as described above was placed toface the counter electrode made of a lithium foil via a separator, andan electrolysis solution was poured therein, so as to prepare a CR2032type coin cell prescribed in IEC60086 for each example. The lithium foilwas a lithium foil (manufactured by Honjo Metal Co., Ltd.) cut out in amanner as to have a diameter of 15 mm and a thickness of 200 μm. Theseparator was Celgard 2400 (manufactured by Celgard, LLC.). Theelectrolysis solution used was prepared in a manner such that LiPF₆(lithium hexafluorophosphate) was dissolved, at a concentration of 1 M,into a mixed non-aqueous solvent in which ethylene carbonate (EC) anddiethyl carbonate (DEC) were mixed in a ratio of 1:1.

[3] Charge-Discharge Test of Batteries

The following charge-discharge test was performed on the respectivecells obtained as described above. That is, the respective cells werecharged and discharged by using a charge-discharge tester in a thermosetbath set at 300 K (27° C.). The charge-discharge tester used wasHJ0501SM8A (manufactured by Hokuto Denko Corporation), and thethermostat bath used was PFU-3K (manufactured by ESPEC Corp.). Each cellwas charged at 0.1 mA from 2 V to 10 mV at the constant current/constantvoltage mode during charging, that is, in the process of Liintercalation to the negative electrode for evaluation. After that, eachcell was discharged at 0.1 mA from 10 mV to 2 V at the constant currentmode during discharge, that is, in the process of Li release from thenegative electrode. This charge-discharge procedure is regarded as asingle charge-discharge cycle. The charge-discharge test was carried outby repeating the above-described charge-discharge cycle 100 times. Then,the discharge capacity maintenance ratios at the 50th cycle and at the100th cycle with respect to the 1st cycle were each analyzed. Table 1shows the results thus obtained. The charge-discharging capacity wascalculated per alloy weight. Note that, in Table 1, “dischargingcapacity maintenance ratio (%)” represents a ratio of the dischargingcapacity at the 50th cycle or at the 100th cycle to the dischargingcapacity at the 1st cycle. Namely, “discharging capacity maintenanceratio (%)” is calculated by ((discharging capacity at 50th cycle or at100th cycle)/(discharging capacity at 1st cycle)×100).

TABLE 1 Discharge Capacity Negative Discharge Maintenance ElectrodeCapacity Ratio (%) Active Material At 1st At At Classi- Component (%)Cycle 50th 100th fication Si Ge Ti (mAh/g) Cycle Cycle Note Example 1 503 47 1700 88 50 Si—Ge—Ti Example 2 31 40 29 1228 87 40 Series Example 321 54 25 932 83 40 Example 4 19 50 31 858 93 42 Example 5 17 63 20 74990 44 Example 6 27 35 38 1197 84 45 Example 7 24 32 44 1086 96 50Example 8 50 16 34 2143 84 42 Example 9 46 15 39 2016 88 47 Example 1039 13 48 1726 83 48 Example 11 37 12 51 1507 93 54 Example 12 34 11 551426 91 51 Example 13 33 10 57 1314 93 53 Example 14 30 10 60 1248 94 53Example 15 29 9 62 1149 93 55 Example 16 27 9 64 1068 94 53 Example 1725 8 67 982 95 50 Example 18 24 8 68 876 93 53 Comparative 100 0 0 323247 16 Pure Si Example 1 Comparative 93 7 0 3827 60 38 S—Ge Example 2Series Comparative 48 52 0 2062 41 26 Example 3 Comparative 39 61 0 173234 22 Example 4 Comparative 33 67 0 1460 26 16 Example 5 Comparative 2872 0 1277 30 18 Example 6 Comparative 0 100 0 1348 80 37 Pure Ge Example7 Comparative 90 0 10 3218 82 36 Si—Ti Example 8 Series Comparative 77 023 2685 82 39 Example 9 Comparative 68 0 32 2398 82 39 Example 10Comparative 60 0 40 2041 83 37 Example 11 Corparative 54 0 46 1784 83 32Example 12 Comparative 49 0 51 1703 75 24 Example 13

As is apparent from Table 1, the batteries of Examples 1 to 18 eachincluding the negative electrode active material containing Si withcontent in the range from greater than or equal to 17% by mass to lessthan 90% by mass, Ti with content in the range from 10% by mass to 83%by mass exclusive, and Ge with content in the range from 0% by mass to73% by mass exclusive, have the initial capacity of greater than orequal to 749 mAh/g. Further, these batteries of the respective examplesshowed the discharge capacity maintenance ratios of 83% or higher at the50th cycle, and 40% or higher even at the 100th cycle. Thus, in view ofensuring a higher capacity and improved cycle durability, the negativeelectrode active material according to the present invention preferablyincludes the alloy containing Si with content in the range from greaterthan or equal to 17% by mass to less than 90% by mass, Ti with contentin the range from 10% by mass to 83% by mass exclusive, and Ge withcontent in the range from 0% by mass to 73% by mass exclusive. Incontrast, the batteries of Comparative Examples 1 to 13 showed thedischarge capacity maintenance ratios that are significantly decreasedcompared with the batteries of Examples even though the dischargecapacities at the 1st cycle are relatively high. Accordingly, the testrevealed that the batteries including the negative electrode activematerial containing the respective components within the specifiedranges according to the present invention can achieve a high capacityand improved cycle durability.

The entire contents of Japanese Patent Application No. P2011-116710(filed on May 25, 2011) is herein incorporated by reference.

Although the present invention has been described above by reference tothe examples, the present invention is not limited to the descriptionsthereof, and it will be apparent to those skilled in the art thatvarious modifications and improvements can be made.

The lithium ion secondary battery is exemplified as an electric devicein the present embodiment and the examples; however, the presentinvention is not limited to the lithium ion secondary battery and isapplicable to secondary batteries of other types and, further, toprimary batteries. In addition, the present invention is applicable notonly to the batteries but also to capacitors. In other words, thenegative electrode for an electric device and the electric deviceaccording to the present invention are only required to contain apredetermined alloy as a negative electrode active material, and otherconstitution requirements are not particularly limited.

The present invention is also applicable to button type batteries andcan type batteries, in addition to the laminated battery describedabove. Further, the present invention is applicable not only to stackingtype (flat-shaped) batteries but also to wound type (cylinder-shaped)batteries. In terms of electrical connection inside the lithium ionsecondary battery, the present invention is applicable not only tointernal parallel connection-type batteries as described above but alsoto internal serial connection-type batteries such as bipolar batteries.Note that, in general, a battery element in a bipolar battery has aconstitution in which bipolar electrodes, each provided with a negativeelectrode active material layer on one side of a current collector and apositive electrode active material layer on the other side of thecurrent collector, and electrolyte layers, are stacked on top of eachother.

INDUSTRIAL APPLICABILITY

The present invention uses, as the negative electrode active materialfor an electric device, the Si—Ge—Ti series ternary alloy. Accordingly,the electric device such as a lithium ion secondary battery employingthe negative electrode active material, contributes to achieving longcycle life and ensuring a high capacity and improved cycle durability.

REFERENCE SIGNS LIST

1 Lithium ion secondary battery

10 Battery element

11 Positive electrode

11A Positive electrode current collector

11B Positive electrode active material layer

12 Negative electrode

12A Negative electrode current collector

12B Negative electrode active material layer

13 Electrolyte layer

14 Single cell layer

21 Positive electrode tab

22 Negative electrode tab

30 Exterior body

1.-9. (canceled)
 10. A negative electrode active material for anelectric device, comprising an alloy containing Si in a range from 17%by mass to 77% by mass inclusive, Ti in a range from greater than orequal to 20% by mass to less than 83% by mass, Ge in a range from 3% bymass to 63% by mass inclusive, and inevitable impurities as a residue.11. The negative electrode active material for an electric deviceaccording to claim 10, wherein the alloy contains Ti of less than orequal to 68% by mass.
 12. The negative electrode active material for anelectric device according to claim 11, wherein the alloy contains Si ofless than or equal to 50% by mass.
 13. The negative electrode activematerial for an electric device according to claim 12, wherein the alloycontains Si of less than or equal to 46% by mass, Ti of greater than orequal to 51% by mass, and Ge of less than or equal to 32% by mass.
 14. Anegative electrode for an electric device, comprising the negativeelectrode active material according to claim
 10. 15. An electric devicecomprising the negative electrode active material according to claim 10.16. An electric device comprising the negative electrode for an electricdevice according to claim
 14. 17. The electric device according to claim15 that is a lithium ion secondary battery.